A decrease in the density of something is rarefaction. As you climb a mountain, you experience rarefaction of the air; the air becomes less dense the higher up you go.

  • Pronunciation: / ,rɛrə'fækʃən/
  • English description: a decrease in the density of something
  • Synonyms: concentration
  • Chinese Translation:  稀薄(xi1 bao2)
  • Spanish Translation: rarefacción
  • ORIGIN: Most of the time, rarefaction refers to air or other gases becoming less dense. When rarefaction occurs, the particles in a gas become more spread out. You may come across this word in the context of sound waves. A sound wave moving through air is made up of alternating areas of higher and lower density. The areas of lower density are called rarefactions.


  • The compressions and rarefactions are rapidly transmitted through the air from the original source as a wave, making sound.
  • Then, too, in extreme states of rarefaction of the sun’s absorbing layer, the absorption of the oxygen is too small to be sensible to us.

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